It's in my TOS that a certain number of revisions are permitted during the sketch/lineart phase; after that a fee is incurred, unless the mistake was my fault. So I wouldn't necessarily have charged for that edit, unless it was after a pile of excessive edits. However, I believe you had every right to do so.
That having been said, I definitely bristled over them pretty much demanding to only pay the 5 bucks and then saying they would edit themselves. Very disrespectful. I don't mind if a patron redlines my stuff to help me along when I'm not getting it, but I would absolutely take offense to that being done without my permission.